CMOS-compatible, refractory conductors are emerging as the materials that will advance novel concepts into real, practical plasmonic technologies. From the available pallet of materials, those with negative real permittivity at very short wavelengths are extremely rare; importantly, they are vulnerable to oxidationupon exposure to far-UV radiationand nonrefractory. Epitaxial, substoichiometric, cubic MoN (B1-MoN x ) films exhibit resistivity as low as 250 μΩ cm and negative real permittivity for experimental wavelengths as short as 155 nm, accompanied with unparalleled chemical and thermal stabilities, which are reported herein. Finite-difference time domain calculations suggest that B1-MoN x operates as an active plasmonic element deeper in the UV (100–200 nm) than any other known material, apart from Al, while being by far more stable and abundant than any other UV plasmonic conductor. Unexpectedly, the unique optical performance of B1-MoN x is promoted by nitrogen vacancies, thus changing the common perception on the role of defects in plasmonic materials.